Transmission device, transmitter, and transmission method

ABSTRACT

An optical transmitter includes a DMT modulating unit that allocates information signals to SCs and that generates a DMT signal by performing multi-level modulation on each of the information signals allocated to each of the SCs. The optical transmitter includes a mixer that shifts, on the basis of the probe result of the DMT signal and frequency information on a wireless signal that is input, the carrier frequency of the wireless signal so as not to overlap the SC to which the information signal in the DMT signal is allocated. Furthermore, the optical transmitter includes a multiplexing unit that multiplexes the DMT signal received from the DMT modulating unit and the wireless signal in which the carrier frequency has been shifted and outputs the multiplexed signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2015-002540, filed on Jan. 8,2015, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a transmission device, atransmitter, and a transmission method.

BACKGROUND

In recent years, for example, communication traffic of wireless signalsof mobile terminals is increased in accordance with an increase inmobile terminals, such as smartphones. Accordingly, by using opticalnetworks, for example, large capacity optical transmission of wirelesssignals of mobile terminals performed among base stations is desired.

Furthermore, in a transmission technique for optical networks, forexample, a multicarrier modulation technique, such as a discretemulti-tone (DMT) modulation technique or the like, is known. The DMTmodulation technique is one of multicarrier transmission technologiesbased on the orthogonal frequency division multiplexing (OFDM)technology. The DMT modulation technique is a technology that allocatesinformation signals to subcarriers (SCs) with different frequencies,performs multi-level modulation on the information signals allocated tothe SCs, and transmits the information signals as DMT signals at a highspeed.

Furthermore, for transmission devices in optical networks, there is aknown method of multiplexing the frequencies of DMT signals intowireless signals of mobile terminals. A transmission device performsfrequency multiplexing on wireless signals into DMT signals, performselectrical to optical conversion on the DMT signals obtained byperforming the frequency multiplexing on the wireless signals, andtransmits optical DMT signals to a transmission device on an oppositeside via an optical network. At this time, the DMT signals use, forexample, the frequency band of several GHz to about 25 GHz, whereas thewireless signals use, for example, the frequency band of about 700 MHzto about 6 GHz. The transmission device on the opposite side separatesthe information signals and the wireless signals from the optical DMTsignals received from the optical network. Consequently, thetransmission device on the opposite side can acquire the informationsignals and the wireless signals from the optical DMT signals.

Patent Document 1: Japanese Laid-open Patent Publication No. 2004-112781

FIG. 15A and FIG. 15B are a schematic diagram illustrating an example ofthe number of bits to be allocated for each SC (frequency) in a DMTsignal. FIG. 15A is a schematic diagram illustrating an example of thenumber of bits to be allocated for each SC in a DMT signal in which awireless signal is not multiplexed. FIG. 15B is a schematic diagramillustrating an example of the number of bits to be allocated for eachSC in a DMT signal in which a wireless signal is multiplexed.Furthermore, FIG. 15A and FIG. 15B illustrate the result of theexperiment obtained when transmitting, by using an optical transmissionline with the length of 10 km, an optical DMT signal obtained bymultiplexing the wireless signal of 2 GHz bandwidth into the DMT signalof 100 Giga Bit Ethernet (GbE (registered trademark)).

In the optical DMT signal indicated in FIG. 15A, from among a pluralityof SCs in the DMT signal, the transmission characteristic of the SCs onthe low frequency side is favorable and the transmission characteristicof the SCs on the high frequency side is gradually degraded.Furthermore, for the DMT signal indicated in FIG. 15A, because thewireless signal of the low frequency band is not subjected to frequencymultiplexing, the transmission characteristic thereof is favorable. Atthis time, the transmission rate is 113 bit per second (Gbps) and thebit error rate (BER) of 6.57×10⁻⁴ is also obtained as the result of theexperiment.

In contrast, in the optical DMT signal indicated in FIG. 15B, becausethe SCs on the low frequency side in which the transmissioncharacteristic thereof is favorable overlap the wireless signal, thetransmission characteristic thereof is remarkably decreased. At thistime, the transmission rate is 109 Gbps and the BER of 8.32×10⁻⁴ is alsoobtained as the result of the experiment.

Namely, for a transmission device, when frequency multiplexing(multiplexing) is performed on a wireless signal into a DMT signal,because the carrier frequency of the wireless signal overlaps the SCs onthe low frequency side in the DMT signal, in which the transmissioncharacteristic is favorable, the transmission characteristic of the DMTsignal is decreased.

SUMMARY

According to an aspect of an embodiment, a transmission device includesa modulating unit, a shifting unit and an output unit. The modulatingunit allocates information signals to a plurality of subcarriers andgenerates a first signal by modulating each of the information signalsallocated to each of the subcarriers. The shifting unit shifts, on thebasis of transmission characteristic information on the first signal andfrequency information on a second signal that is input, the carrierfrequency of the second signal so as not to overlap the subcarriers towhich the information signals in the first signal are allocated. Theoutput unit multiplexes the first signal and the second signal shiftedby the shifting unit and outputs the multiplexed signal.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example of an opticaltransmission system according to a first embodiment;

FIG. 2 is a schematic diagram illustrating an example of a wirelesstechnique table;

FIG. 3 is a schematic diagram illustrating an example of an allocationtable;

FIG. 4 is a schematic diagram illustrating an example of the frequencyspectrum of a wireless signal and a DMT signal;

FIG. 5 is a schematic diagram illustrating an example of the proberesult (SNR-frequency characteristic) of the DMT signal;

FIG. 6 is a schematic diagram illustrating an example of the number ofbits to be allocated for each SC (frequency) in the DMT signal;

FIG. 7 is a schematic diagram illustrating an example of the SCs in theDMT signal that is obtained by multiplexing a wireless signal;

FIG. 8 is a flowchart illustrating an example of an operation of aprocess performed by an optical transmitter related to an opticaltransmission process;

FIG. 9 is a flowchart illustrating an example of an operation of aprocess performed by an allocating unit on the optical transmitter siderelated to an allocation process;

FIG. 10 is a schematic diagram illustrating an example of an operationof multiplexing and demultiplexing related to the DMT signal;

FIG. 11 is a block diagram illustrating an example of an opticaltransmission system according to a second embodiment;

FIG. 12 is a flowchart illustrating an example of an operation of aprocess performed by an optical transmitter related to the opticaltransmission process;

FIG. 13 is a block diagram illustrating an example of an opticaltransmission system according to a third embodiment;

FIG. 14 is a schematic diagram illustrating a transmission device thatexecutes a transmission program; and

FIG. 15A and FIG.15B are a schematic diagram illustrating an example ofthe number of bits to be allocated for each SC (frequency) in a DMTsignal.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained withreference to accompanying drawings. The present invention is not limitedto these embodiments.

[a] First Embodiment

FIG. 1 is a block diagram illustrating an example of an opticaltransmission system according to a first embodiment. An opticaltransmission system 1 illustrated in FIG. 1 includes an opticaltransmitter 10, an optical receiver 20, and an optical transmission line2 that connects between the optical transmitter 10 and the opticalreceiver 20. The optical transmitter 10 is, for example, a transmissiondevice, such as an optical module or the like, that is connected tooptical access network in which mobile terminals are connected to basestations that accommodate the mobile terminals. The mobile terminalsmentioned here are, for example, wireless terminals, such as, mobilephones, smartphones, tablet terminals, or the like. The optical receiver20 mentioned here is, for example, a transmission device, such as anoptical module or the like, that is accommodated in an optical metronetwork, an optical core network, or the like.

The optical transmitter 10 includes a DMT modulating unit 11, amultiplexing unit 12, an electrical to optical conversion unit(hereinafter, simply referred to as an E/O) 13, a branching unit 14, amixer 15, an oscillator 16, a monitoring unit 17, and an allocating unit18. The DMT modulating unit 11 is, for example, a modulating unit thatallocates information signals to subcarriers (hereinafter, simplyreferred to as SCs) in channels having different frequencies, performsmulti-level modulation on the information signal allocated for each SC,and generates a DMT signal. The branching unit 14 inputs, for example, awireless signal of each of the mobile terminals from a base station andinputs, to the mixer 15 and the monitoring unit 17, each of the inputwireless signals. The mixer 15 performs frequency conversion on thewireless signal received from the branching unit 14 in accordance withthe oscillating frequency from the oscillator 16 and shifts the carrierfrequency of the wireless signal. Furthermore, the mixer 15 inputs theshifted wireless signal to the multiplexing unit 12. The multiplexingunit 12 multiplexes the wireless signal received from the mixer 15 intothe DMT signal received from the DMT modulating unit 11. The E/O 13performs electrical to optical conversion on the DMT signal that isobtained by multiplexing the wireless signal and then outputs theoptical DMT signal to the optical transmission line 2 for transmission.Furthermore, the multiplexing unit 12 and the E/O 13 correspond to, forexample, an output unit.

The monitoring unit 17 includes a spectrum analyzer 17A and a wirelesstechnique table 31. The spectrum analyzer 17A analyzes the frequencyspectrum of the wireless signal received from the branching unit 14 andacquires, for example, a peak frequency from the result of the subjectanalysis. Furthermore, the monitoring unit 17 refers to the wirelesstechnique table 31 on the basis of the analysis result of the wirelesssignal and acquires the frequency information, such as a wirelesstechnique or the like of the wireless signal, that was input by thebranching unit 14. FIG. 2 is a schematic diagram illustrating an exampleof the wireless technique table 31. The wireless technique table 31illustrated in FIG. 2 manages, in an associated manner for each of asignaling technique 31A of a wireless signal, a carrier frequency 31Band a use band width 31C. The signaling technique 31A is a signalingtechnique for a wireless signal and is, for example, a signalingtechnique, such as wideband code division multiple access (W-CDMA) orlong term evolution (LTE, registered trademark, the same also applies tothe description below), WiMAX (registered trademark), or the like.Furthermore, even in the same signaling technique, management isindividually performed for multiple telecommunications carriers. Thecarrier frequency 31B is a carrier frequency used by a signalingtechnique. The use band width 31C is the frequency band used in thesignaling technique. The monitoring unit 17 refers to the wirelesstechnique table 31 and acquires the frequency information, such as thesignaling technique 31A, the carrier frequency 31B, and the use bandwidth 31C of the wireless signal, that is associated with the peakfrequency of the wireless signal.

The allocating unit 18 determines, on the basis of the carrier frequencyand the use band width associated with the signaling technique for thewireless signal obtained from the monitoring unit 17, the SCs to beallocated to the wireless signal and the information signal from among aplurality of SCs in the DMT signal. The allocating unit 18 includes anallocation table 32. FIG. 3 is a schematic diagram illustrating anexample of the allocation table 32. The allocation table 32 illustratedin FIG. 3 manages, in an associated manner for each of an SC number 32A,a use frequency 32B and an allocation signal 32C. The SC number 32A isan identification number for identifying an SC. The use frequency 32B isthe frequency of the SC that is used for each of the SC number 32A. Theallocation signal 32C is the signal type, such as “wireless” of awireless signal or “DMT” of an information signal, that is allocated tothe SC with the SC number 32A.

The allocating unit 18 acquires, from the monitoring unit 17, the errorvector magnitude (target EVM) that is associated with the wirelesstechnique for the wireless signal. Furthermore, the target EVM is, forexample, a recommended evaluation value of a wireless signal that isrecommended by telecommunications carriers. The allocating unit 18calculates a signal to noise ratio (SNR) of a wireless signal by usingthe formula of SNR (dB)=−[PAPR+20×log₁₀(EVM (%))]. For example, when thewireless signal is a signal with quadrature amplitude modulation (QAM)of 64, when the peak to average power ratio (PAPR) thereof is 3.7 dB,and when the target EVM is 8%, the SNR thereof is 18.2 dB.

FIG. 4 is a schematic diagram illustrating an example of the frequencyspectrum of a wireless signal and a DMT signal. As illustrated in FIG.4, the allocating unit 18 calculates a signal level difference that is adifference between the signal level related to the carrier frequency ofthe wireless signal and the signal level related to the DMT signal ofthe subject carrier frequency. The signal level difference illustratedin FIG. 4 is 16 dB. The allocating unit 18 calculates a conversion SNRby subtracting the signal level difference from the SNR of the wirelesssignal of the target EVM. For example, the allocating unit 18 calculatesthe conversion SNR as the result of 18.2 dB−16 dB=2.2 dB.

FIG. 5 is a schematic diagram illustrating an example of the proberesult (SNR-frequency characteristic) of the DMT signal. The allocatingunit 18 searches the probe result of the DMT signal illustrated in FIG.5 for the frequency that is associated with the conversion SNR anddetermines the frequency of the search result as the shifted carrierfrequency of the wireless signal. Furthermore, the probe result of theDMT signal is a transmission characteristic of the DMT signal obtainedfrom the result of transmission of a test signal of a DMT signal by theoptical transmitter 10 itself before an operation. Namely, theallocating unit 18 searches the probe result illustrated in FIG. 5 forthe frequency of 27 GHz that is associated with the conversion SNR of2.2 dB and determines the shifted carrier frequency of the wirelesssignal as 27 GHz. After the allocating unit 18 determines the shiftedcarrier frequency of the wireless signal, the allocating unit 18calculates a frequency shift amount between the carrier frequency F1 ofthe current wireless signal and the shifted carrier frequency F2 andthen sets the frequency shift amount in the oscillator 16. FIG. 6 is aschematic diagram illustrating an example of the number of bits to beallocated for each SC (frequency) in the DMT signal. The wireless signalillustrated in FIG. 6 is shifted from the SCs on the low frequency sidehaving a favorable transmission characteristic, in which the number ofbits to be allocated to the DMT signal is large, to the SCs on the highfrequency side having a transmission characteristic in which the numberof bits to be allocated is small. The mixer 15, the oscillator 16, themonitoring unit 17, and the allocating unit 18 correspond to, forexample, a shifting unit.

FIG. 7 is a schematic diagram illustrating an example of the SCs in theDMT signal that is obtained by multiplexing a wireless signal. The DMTsignal illustrated in FIG. 7 includes a wireless signal 71, guard bands72, and information signals 73. The allocating unit 18 acquires, fromthe monitoring unit 17, the carrier frequency fc and the use band widthAf that are associated with the signaling technique for the wirelesssignal 71. Furthermore, it is assumed that the use band width of theguard band is Δfg.

As illustrated in FIG. 7, the allocating unit 18 calculates, on thebasis of the frequency bands of fc−Δf/2 to fc+Δf/2 of the wirelesssignal, a guard band on a lower side of fc−Δf/2−Δfg and a guard band ona higher side of fc+Δf/2+Δfg. Namely, the allocating unit 18 calculatesthe guard band on the lower side and the guard band on the higher siderelated to the wireless signal of the determined shifted frequency of Fc(=fc+X).

Furthermore, the allocating unit 18 searches the SC numbers on the lowerside of the frequency band of f<Fc−Δf/2−Δfg for a maximum SC number N1that is present, in a direction in which the frequency is decreased,subsequent to the SC numbers on the lower side. The allocating unit 18searches the SC numbers on the higher side of the frequency band off>Fc+Δf/2+Δfg for a minimum SC number N2 that is present, in a directionin which the frequency is increased, subsequent to the SC numbers on thehigher side.

The allocating unit 18 allocates the information signal of “DMT” to theallocation of the maximum SC number N1 that is present, in a directionin which the frequency is decreased, subsequent to the SC numbers on thelower side and allocates the information signal of “DMT” to theallocation of the minimum SC number N2 that is present, in a directionin which the frequency is increased, subsequent to the SC numbers on thehigher side. Furthermore, the allocating unit 18 allocates the wirelesssignal of “wireless” to the allocation of the SC numbers of the SCspresent in the range between the SC that has the SC number N1+1 and thatis adjacent, in the direction in which the frequency is increased, tothe SC number N1 on the lower side and the SC that has the SC numberN2−1 and that is adjacent, in the direction in which the frequency isdecreased, to the SC number N2 on the higher side.

The allocating unit 18 updates, on the basis of the allocation content,the allocation information of the SC number 32A in the allocation table32. The allocating unit 18 refers to the allocation table 32 and sets,in the DMT modulating unit 11, the allocation information, such as theuse frequency 32B, the allocation signal 32C, and the like, that areassociated with the SC number 32A. The DMT modulating unit 11 generates,on the basis of the allocation information, a DMT signal in which theinformation signals are allocated to the SCs that do not overlap thecarrier frequency of the wireless signal in which the frequency has beenshifted. The allocating unit 18 notifies the optical receiver 20 via,for example, a control line 3, of the setting information to which, inaddition to the allocation information, frequency information includingcarrier frequencies of the wireless signals before the shift and afterthe shift, the use band width, or the like are added.

The optical receiver 20 includes an optical to electrical convertingunit (hereinafter, simply referred to as an O/E) 21, a demultiplexingunit 22, a DMT demodulating unit 23, a mixer 24, and an oscillator 25.The O/E 21 performs electric conversion on the optical DMT signalreceived from the optical transmission line 2 and obtains a DMT signal.On the basis of carrier frequency Fc of the shifted wireless signal andthe use band width Δf in the setting information received from theoptical transmitter 10, the demultiplexing unit 22 separates the shiftedwireless signal and the DMT signal from the DMT signal. The oscillator25 generates an oscillating frequency that is associated with thecarrier frequency before the frequency has not been shifted and that isstored in the setting information received from the allocating unit 18on the optical transmitter 10 side. Furthermore, the mixer 24 performsfrequency conversion on the wireless signal in accordance with theoscillating frequency output from the oscillator 25, restores thewireless signal to the carrier frequency before the frequency has notbeen shifted, and outputs the wireless signal before the frequency hasnot been shifted. Furthermore, on the basis of the allocationinformation received from the allocating unit 18 in the opticaltransmitter 10, the DMT demodulating unit 23 demodulates the separatedDMT signal and outputs the information signals.

In the following, an operation of the optical transmission system 1according to the first embodiment will be described. FIG. 8 is aflowchart illustrating an example of an operation of a process performedby the optical transmitter 10 related to an optical transmissionprocess. The optical transmission process illustrated in FIG. 8 is aprocess of shifting the carrier frequency of the wireless signal to theSCs on the high frequency side in the DMT signal, multiplexing thewireless signal into the DMT signal, and transmitting the signal to theoptical transmission line 2.

In FIG. 8, the monitoring unit 17 in the optical transmitter 10acquires, via the spectrum analyzer 17A, a frequency spectrum of thepeak frequency or the like of the wireless signal (Step S11).Furthermore, the monitoring unit 17 refers to the wireless techniquetable 31 and determines, on the basis of the acquired frequencyspectrum, a frequency technique, a carrier frequency, and a frequencyband of the wireless signal (Step S12). Furthermore, the allocating unit18 in the optical transmitter 10 acquires, as illustrated in FIG. 5, thetransmission characteristic of the DMT signal from the probe result ofthe DMT signal (Step S13). The allocating unit 18 calculates an SNR ofthe wireless signal that is associated with the target EVM associatedwith the wireless technique (Step S14). Furthermore, the allocating unit18 calculates an SNR of the wireless signal on the basis of SNR(dB)=−[PAPR+20×log₁₀(EVM(%))]. The allocating unit 18 acquires a signallevel that is associated with the number of bits to be allocated foreach frequency (SC) in the DMT signal (Step S15).

The allocating unit 18 subtracts the signal level of the DMT signalassociated with the carrier frequency of the wireless signal from thesignal level associated with the subject carrier frequency of thewireless signal and calculates a signal level difference therebetween(Step S16). After the allocating unit 18 calculates the signal leveldifference, the allocating unit 18 subtracts the signal level differencefrom the SNR of the wireless signal of the target EVM and calculates aconversion SNR (Step S17). The allocating unit 18 determines whether theconversion SNR is equal to or less than 0 (Step S18). When theconversion SNR is not equal to or less than 0 (No at Step S18), theallocating unit 18 searches the transmission characteristic of the DMTsignal, i.e., the probe result illustrated in FIG. 5, for the frequencyassociated with the conversion SNR (Step S19).

The allocating unit 18 determines the frequency of the search result asthe shifted carrier frequency of the wireless signal (Step S20). Theallocating unit 18 calculates, on the basis of the determined shiftedcarrier frequency of the wireless signal, a frequency shift amount fromthe current carrier frequency to the shifted carrier frequency of thewireless signal (Step S21). The allocating unit 18 sets the calculatedfrequency shift amount in the oscillator 16 (Step S22). Then, theallocating unit 18 performs an allocation process illustrated in FIG. 9,which will be described later (Step S23).

Furthermore, after performing the allocation process, the DMT modulatingunit 11 generates a DMT signal on the basis of the allocationinformation in the setting information (Step S24). The mixer 15 performsfrequency conversion on the wireless signal in accordance with theoscillating frequency having the frequency shift amount from theoscillator 16 and then shifts the carrier frequency of the wirelesssignal (Step S25). Consequently, the wireless signal is shifted to thecarrier frequency determined at Step S20 or Step S29. The multiplexingunit 12 multiplexes the wireless signal received from the mixer 15 intothe DMT signal received from the DMT modulating unit 11 (Step S26).Furthermore, the E/O 13 converts the multiplexed DMT signal to anoptical DMT signal (Step S27), transmits the optical DMT signal to theoptical transmission line 2 (Step S28), and ends the operation of theprocess illustrated in FIG. 8. Furthermore, the optical DMT signal isthe optical signal that is obtained by shifting the carrier frequency ofthe wireless signal to the SCs on the high frequency side in the DMTsignal and multiplexing the wireless signal into the DMT signal. For theDMT signal, the carrier frequency of the wireless signal does notoverlap the SCs on the low frequency side, which means that the subjecttransmission characteristic has been improved.

Furthermore, when the conversion SNR is equal to or less than 0 (Yes atStep S18), the allocating unit 18 determines the frequency outside theDMT signal band as the shifted carrier frequency of the wireless signal(Step S29). The frequency outside the DMT signal band mentioned here is,for example, the frequency band on the high frequency side or thefrequency band on the low frequency side that is not used by the DMTsignal. Then, the allocating unit 18 proceeds to Step S21 in order tocalculate a frequency shift amount on the basis of the shifted carrierfrequency of the determined wireless signal.

When the conversion SNR is not equal to or less than 0, the opticaltransmission process illustrated in FIG. 8 searches the transmissioncharacteristic of the DMT signal for the frequency associated with theconversion SNR, determines the frequency as the shifted carrierfrequency of the wireless signal, and sets the wireless signal to theSCs on the high frequency side in the DMT signal. Consequently, becausethe wireless signal does not overlap the SCs on the low frequency sidein the DMT signal, the transmission characteristic of the DMT signal canbe improved.

When the conversion SNR is equal to or less than 0, the opticaltransmission process determines the frequencies that are other than theuse band of the DMT signal and that are not used as the carrierfrequencies of the wireless signal and sets the wireless signal to theband other than the use band of the DMT signal. Consequently, thewireless signal does not overlap the use band of the DMT signal, thetransmission characteristic of the DMT signal can be improved.

FIG. 9 is a flowchart illustrating an example of an operation of aprocess performed by the allocating unit 18 on the optical transmitter10 side related to an allocation process. The allocation processillustrated in FIG. 9 is a process of setting the allocation of thewireless signal and the information signal of the SC in the DMT signalso as not to overlap the shifted carrier frequency of the determinedwireless signal.

In FIG. 9, on the basis of the carrier frequency fc of the wirelesssignal acquired by the monitoring unit 17 and on the basis of thecalculated frequency shift amount X, the allocating unit 18 calculates acarrier frequency Fc of the shifted wireless signal (Step S41). Theallocating unit 18 searches the SCs in the DMT signal for the maximum SCnumber N1 that satisfies the condition of f<Fc−Δf/2−Δfg (Step S42).Furthermore, the maximum SC number N1 is the SC that is adjacent to thecarrier frequency of the shifted wireless signal and that is present inthe DMT signal in a direction in which the frequency is decreased. Theallocating unit 18 searches the SCs in the DMT signal for the minimum SCnumber N2 that satisfies the condition of f>Fc+Δf/2+Δfg (Step S43).Furthermore, the minimum SC number N2 is the SC that is adjacent to thecarrier frequency of the shifted wireless signal and that is present inthe DMT signal in a direction in which the frequency is increased.

The allocating unit 18 refers to the allocation table 32 and determineswhether the allocation of the SC number N with the condition of N1≦N≦N2is a wireless signal (Step S44). When the allocation of the SC number Nis a wireless signal (Yes at Step S44), the allocating unit 18 refers tothe allocation table 32 and determines whether the allocation of the SCnumber N with the conditions of Nl≦N and N≦N2 is an information signal(Step S45).

When the allocation of the SC number N is an information signal (Yes atStep S45), the allocating unit 18 sets, in the DMT modulating unit 11,the allocation information including the allocation content for each SCnumber (Step S46). After the allocating unit 18 sets the allocationinformation in the DMT modulating unit 11, the allocating unit 18notifies the DMT demodulating unit 23 and the demultiplexing unit 22 onthe optical receiver 20 side of the allocation information (Step S47)and ends the operation of the process illustrated in FIG. 9.Furthermore, when the allocation of the SC number N with the conditionof N1≦N≦N2 is not a wireless signal (No at Step S44), the allocatingunit 18 sets the allocation of the SC number N to the wireless signaland updates the content of the allocation table 32 (Step S48). Then, theallocating unit 18 proceeds to Step S45 in order to determine whetherthe allocation of the SC number N is an information signal.

When the allocation of the SC number N with the conditions of N1≦N andN≦N2 is not an information signal (No at Step S45), the allocating unit18 sets the allocation of the SC number N with the conditions of N1≦Nand N≧N2 to the information signal (DMT) and updates the content of theallocation table 32 (Step S49). Then, the allocating unit 18 proceeds toStep S46 in order to set the setting information in the DMT modulatingunit 11.

In the allocation process illustrated in FIG. 9, after the carrierfrequency of the shifted wireless signal is determined, the SCsassociated with the shifted carrier frequency of the wireless signal inthe DMT signal are allocated to a wireless signal. Furthermore, the SCsother than the SCs associated with the shifted carrier frequency of thewireless signal in the DMT signal are allocated to the informationsignal (DMT). Consequently, the information signal in the DMT signaldoes not overlap the wireless signal and thus the degradation of thetransmission characteristic of the DMT signals and the wireless signalscan be suppressed. Namely, the optical transmitter 10 efficientlytransmits the wireless signals by using the optical DMT signals.

FIG. 10 is a schematic diagram illustrating an example of an operationof multiplexing and demultiplexing related to the DMT signal. Asindicated by (A) illustrated in FIG. 10, the mixer 15 in the opticaltransmitter 10 shifts the carrier frequency of the wireless signal tothe SCs on the high frequency side in the DMT signal. Furthermore, theDMT modulating unit 11 in the optical transmitter 10 generates, on thebasis of the allocation information, a DMT signal in which theinformation signals are allocated to the SCs such that the informationsignals are not allocated to the SCs that are associated with theshifted carrier frequency of the wireless signal. Namely, because theDMT modulating unit 11 has shifted the wireless signal to the highfrequency side, the information signals can be allocated to the SCs onthe low frequency in which the transmission characteristic is favorable.Then, the multiplexing unit 12 in the optical transmitter 10 multiplexesthe wireless signal received from the mixer 15 into the DMT signalreceived from the DMT modulating unit 11 and transmits, as indicated by(B) illustrated in FIG. 10, the optical DMT signal to the opticalreceiver 20. Consequently, because the wireless signal does not overlapthe information signals allocated to the SCs on the low frequency sidein the DMT signal in which the transmission characteristic is favorable,the transmission characteristic of the DMT signal can be improved.

Furthermore, when the demultiplexing unit 22 on the optical receiver 20side receives an optical DMT signal, as indicated by (C) illustrated inFIG. 10, the demultiplexing unit 22 separates the DMT signal and thewireless signal from the optical DMT signal. The DMT demodulating unit23 on the optical receiver 20 side modulates the DMT signal and outputsthe information signal. Furthermore, the mixer 24 on the opticalreceiver 20 side can restore, on the basis of the frequency information,the wireless signal that is input from the demultiplexing unit 22 to thecarrier frequency of the SCs on the low frequency before the shift isperformed and output the wireless signal of the carrier frequency beforethe frequency has not been shifted. Consequently, the optical receiver20 can acquire not only the information signal but also the wirelesssignal from the optical DMT signal received from the optical transmitter10.

The optical transmitter 10 according to the first embodiment shifts thecarrier frequency of the wireless signal to the SCs on the highfrequency side in the DMT signal such that the information signal andthe wireless signal in the DMT signal do not overlap the SCs on the lowfrequency side, multiplexes the wireless signal into the DMT signal, andthen transmits the signal to the optical transmission line 2.Consequently, because the information signal and the wireless signal inthe DMT signal do not overlap the SCs on the low frequency side, thetransmission characteristic of the subject DMT signal can be improved.

The optical transmitter 10 calculates a conversion SNR by subtractingthe signal level difference between the wireless signal and the DMTsignal from the SNR of the wireless signal that satisfies the target EVMand then searches the probe result of the DMT signal for the frequencycorresponding to the conversion SNR. Furthermore, the opticaltransmitter 10 determines the frequency of the search result as theshifted carrier frequency of the wireless signal. Consequently, theshifted carrier frequency that satisfies the target EVM of the wirelesssignal can be acquired.

Furthermore, the allocating unit 18 according to the first embodimentdescribed above calculates a signal level difference between the signallevel of the carrier frequency of the wireless signal and the signallevel of the DMT signal associated with the carrier frequency of thewireless signal and calculates a conversion SNR on the basis of thecalculated signal level difference and the SNR of the wireless signal ofthe target EVM. However, when the signal level difference is small, thefrequency shift amount is also small and thus the improvement effect ofthe transmission characteristic of the DMT signal is also small.Accordingly, an embodiment of this case will be described below as asecond embodiment.

[b] Second Embodiment

FIG. 11 is a block diagram illustrating an example of an opticaltransmission system 1A according to a second embodiment. By assigningthe same reference numerals to components having the same configurationas those in the optical transmission system 1 according to the firstembodiment, descriptions thereof will be omitted.

The optical transmission system 1A illustrated in FIG. 11 differs fromthe optical transmission system 1 illustrated in FIG. 1 in that anamplifier 19 is disposed upstream of the branching unit 14 in an opticaltransmitter 10A and, when the signal level difference is less than apredetermined threshold, the level of the wireless signal is amplifiedby the amplifier 19 and the signal is input to the branching unit 14.The amplifier 19 corresponds to, for example, an adjustment unit.

An allocating unit 18A determines whether the signal level differencebetween the signal level of the carrier frequency of the wireless signaland the signal level of the DMT signal is less than the predeterminedthreshold. When the signal level difference is less than thepredetermined threshold, the allocating unit 18A requests the amplifier19 to amplify the signal level of the wireless signal.

The amplifier 19 amplifies, in accordance with the amplification requestfrom the allocating unit 18A, the signal level of the input wirelesssignal and then inputs the amplified wireless signal to the branchingunit 14. Then, the monitoring unit 17 acquires the frequency spectrum ofthe wireless signal via the spectrum analyzer 17A. At this time, themonitoring unit 17 acquires the signal level of the wireless signalamplified by the amplifier 19.

Because the signal level of the carrier frequency of the wireless signalis amplified, the allocating unit 18A can secures a signal leveldifference equal to or greater than a predetermined threshold as thedifference between the signal level of the wireless signal and thesignal level of the DMT signal.

In the following, an operation of the optical transmission system 1Aaccording to the second embodiment will be described. FIG. 12 is aflowchart illustrating an example of an operation of a process performedby the optical transmitter 10A related to the optical transmissionprocess. In FIG. 12, the allocating unit 18A calculates the signal leveldifference at Step S16. The allocating unit 18A determines whether thesignal level difference is less than the predetermined threshold (StepS31). When the signal level difference is less than the predeterminedthreshold (Yes at Step S31), the allocating unit 18A requests theamplifier 19 to amplify the wireless signal by a predetermined amount(Step S32). Consequently, the amplifier 19 amplifies the signal level ofthe wireless signal in accordance with the amplification request fromthe allocating unit 18A.

Furthermore, after the allocating unit 18A requests the amplification bythe predetermined amount, the allocating unit 18A proceeds to Step S16in order to calculate a signal level difference between the wirelesssignal that is associated with the carrier frequency of the wirelesssignal and the DMT signal. Consequently, the signal level differencebetween the wireless signal and the DMT signal becomes large by anamount corresponding to the amplified signal level of the wirelesssignal.

Furthermore, when the signal level difference is not less than thepredetermined threshold (No at Step S31), the allocating unit 18Aproceeds to Step S17 in order to calculate a conversion SNR of thewireless signal on the basis of the SNR of the wireless signal of thetarget EVM and the signal level difference.

The optical transmission process illustrated in FIG. 12 amplifies, whenthe signal level difference between the wireless signal and the DMTsignal is less than the predetermined threshold, the wireless signal andagain calculates a signal level difference between the wireless signaland the DMT signal. Consequently, it is possible to secure the signallevel difference having a value equal to or greater than thepredetermined threshold and secure the conversion SNR that is used whenthe carrier frequency of the wireless signal is shifted.

When the signal level difference between the wireless signal and the DMTsignal is less than the predetermined threshold, the opticaltransmission system 1A according to the second embodiment amplifies thewireless signal and again calculates a signal level difference betweenthe wireless signal and DMT signal. Consequently, it is possible tosecure the signal level difference having the value equal to or greaterthan the predetermined threshold and acquire the conversion SNR that isused when the carrier frequency of the wireless signal is shifted.

When the conversion SNR is not equal to or less than 0, the opticaltransmitter 10A searches the transmission characteristic of the DMTsignal for the frequency associated with the subject conversion SNR,determines the frequency as the carrier frequency of the shiftedwireless signal, and sets the wireless signal to the SCs on the highfrequency side in the DMT signal. Consequently, because the wirelesssignal does not overlap the SCs on the low frequency side in the DMTsignal, the transmission characteristic of the DMT signal can beimproved.

When the conversion SNR is equal to or less than 0, the opticaltransmitter 10A determines, as the carrier frequency of the wirelesssignal, the frequency that is other than the use band of the DMT signaland that has not been used and sets the wireless signal to the bandother than the use band of the DMT signal. Consequently, because thewireless signal does not overlap the use band of the DMT signal, thetransmission characteristic of the DMT signal can be improved.

Furthermore, in the first and the second embodiments described above,for convenience of explanation, the optical transmission system 1 (1A)constituted by the optical transmitter 10 (10A) and the optical receiver20 has been described; however, the optical transmitter 10 (10A) and theoptical receiver 20 may also be embedded in a single transmissiondevice. The embodiment according to this case will be described below asa third embodiment.

[c] Third Embodiment

FIG. 13 is a block diagram illustrating an example of an opticaltransmission system 1B according to a third embodiment. By assigning thesame reference numerals to components having the same configuration asthose in the optical transmission system 1 according to the firstembodiment, descriptions thereof will be omitted.

The optical transmission system 1B illustrated in FIG. 13 includes anoptical transmission device 5A (5) and an optical transmission device 5B(5) that is disposed on the opposite side and is configured such thatthe optical transmission device 5A and the optical transmission device5B that is disposed on the opposite side are connected by the opticaltransmission line 2. Furthermore, the optical transmission device 5B onthe opposite side has the same configuration as that of the opticaltransmission device 5A; therefore, by assigning the same referencenumerals to components having the same configuration, descriptionsthereof will be omitted.

The transmission device 5 includes an optical transmitter 10B and anoptical receiver 20B. The optical transmitter 10B includes the DMTmodulating unit 11, the multiplexing unit 12, the E/O 13, the branchingunit 14, the mixer 15, the oscillator 16, the monitoring unit 17, andthe allocating unit 18. The optical receiver 20B includes the O/E 21,the demultiplexing unit 22, the DMT demodulating unit 23, and the mixer24. Furthermore, the oscillator on the optical receiver 20B shares theoscillator 16 in the optical transmitter 10B.

The optical transmission device 5A transmits, by using the opticaltransmission line 2 between the optical transmission device 5B disposedon the opposite side, an optical DMT signal that is obtained bymultiplexing the DMT signal into the wireless signal.

Because the transmission devices 5 (5A and 5B) according to the thirdembodiment include therein the optical transmitter 10B and the opticalreceiver 20B and, furthermore, the oscillator 16 is shared by theoptical transmitter 10B and the optical receiver 20B, the number ofcomponents can be reduced.

In the first to the third embodiments, a case in which the allocatingunit 18 is embedded in the optical transmitter 10 (10A and 10B) has beendescribed as an example; however, the allocating unit 18 may also beembedded in the optical receiver 20 (20B) or may also be embedded in amanagement device other than the optical transmitter 10 and the opticalreceiver 20.

Furthermore, a description has been given of a case in which theallocating unit 18 shifts the carrier frequency of the wireless signalthat is multiplexed into the DMT signal to the SC on the high frequencyside in the DMT signal; however, the band that is not used by the SC inthe DMT signal may be used. For example, the carrier frequency of thewireless signal may also be shifted to the frequency associated with afree SC in the DMT signal such that the information signal and wirelesssignal in the DMT signal do not overlap.

Furthermore, the allocating unit 18 includes therein the allocationtable 32 that manages the allocation content for each SC in the DMTsignal. However, instead of preparing the allocation table 32, theallocation content of the SC in the DMT signal may be detected each timeand the allocation process may also be performed.

In the first to the third embodiments described above, the settinginformation is notified from the optical transmitter 10 to the opticalreceiver 20 via the control line 3 that is different from the opticaltransmission line 2. However, for example, the allocation informationmay also be included in a control signal, for example, an opticalservice channel (OSC) signal, of the multicarrier signal transmitted inthe optical transmission line 2 and then sent as a notification from theoptical transmitter 10 to the optical receiver 20.

In the first to the third embodiments described above, electrical tooptical conversion is performed after electrically multiplexing thewireless signal into the DMT signal; however, after performingelectrical to optical conversion on the DMT signal and the wirelesssignal, optical frequency multiplexing may also be performed on the DMTsignal and the wireless signal that have been subjected to theelectrical to optical conversion. Consequently, this method is effectivein signal processing using the high frequency band when compared with acase in which multiplexing (frequency multiplexing) is performed byusing an electrical signal.

Furthermore, in the embodiments described above, the optical transmitter10 that uses the DMT modulation technique is described as an example;however, the embodiment may also be applied to an optical transmitterthat uses another multicarrier modulation technique that allocatesinformation signals to multiple SCs and that modulates each of theinformation signals allocated to each of the SCs. For example, theembodiment may also be applied to an optical transmitter that uses themulticarrier modulation technique, such as various kinds PSK modulationtechniques including the Orthogonal Frequency Division Multiplexing(OFDM) modulation technique or the Quadrature Phase Shift Keying (QPSK)modulation technique. Similarly, the embodiment may also be applied tothe differential phase shift keying (DPSK), 8-PSK, or the like.

Furthermore, the components of each unit illustrated in the drawings arenot always physically configured as illustrated in the drawings. Inother words, the specific shape of a separate or integrated unit is notlimited to the drawings; however, all or part of the unit can beconfigured by functionally or physically separating or integrating anyof the units depending on various loads or use conditions.

Furthermore, all or any part of various kinds of processing functionsperformed by each unit may also be executed by a central processing unit(CPU), a digital Signal processor (DSP), a field programmable gate array(FPGA), or the like. Furthermore, all or any part of the various kindsof processing functions may also be executed by programs analyzed andexecuted by the CPU and the like or executed by hardware by wired logic.

The area in which various kinds of information is stored is, forexample, a read only memory (ROM) or a random access memory (RAM), suchas a synchronous dynamic random access memory (SDRAM), amagnetoresistive random access memory (MRAM), a non volatile memory(NVRAM), or the like.

The various processes described in the embodiments can be implemented bya program prepared in advance and executed by an optical module.Accordingly, in the following, an example of a transmission device, suchas the optical module, that executes a program having the same functionas that performed in the embodiments described above. FIG. 14 is aschematic diagram illustrating a transmission device 100 that executes atransmission program.

In FIG. 14, a transmission device 100 that executes a transmissionprogram includes a communication interface 110, a ROM 120, a RAM 130,and a processor 140.

The ROM 120 stores therein, in advance, a transmission program havingthe same function as that performed in the embodiments described above.Furthermore, instead of the ROM 120, the transmission program may alsobe stored in a recording medium that can be read by a drive (notillustrated). Furthermore, the recording medium may also be, forexample, a portable recording medium, such as a CD-ROM, a DVD disk, aUSB memory, an SD card, or the like, or a semiconductor memory, such asa flash memory, or the like. Furthermore, the transmission program mayalso be acquired from a storage device that can be communicated via anetwork. As illustrated in FIG. 14, a modulation program 120A, a shiftprogram 120B, and an output program 120C are used as the transmissionprogram. Furthermore, the programs 120A, 120B, and 120C may alsoappropriately be integrated or separated.

Then, the processor 140 reads the programs 120A, 120B, and 120C from theROM 120 and executes each of the read programs on the RAM 130. Then, theprocessor 140 allows the programs 120A, 120B, and 120C to function, onthe RAM 130, as a modulation process 130A, a shift process 130B, and anoutput process 130C, respectively.

The processor 140 allocates information signals to a plurality ofsubcarriers, modulates each of the information signals allocated to eachof the subcarriers, and generates a first signal. The processor 140shifts, on the basis of the transmission characteristic information onthe first signal and the frequency information on an input secondsignal, the carrier frequency of a second signal so as not to overlapthe subcarriers to which the information signals in the first signal areallocated. The processor 140 multiplexes the first signal and theshifted second signal and outputs the multiplexed signal. Consequently,even when the first signal is multiplexed into the second signal, thetransmission characteristic of the first signal can be improved.

According an aspect of an embodiment, an advantage is provided in thatthe transmission characteristic of the first signal can be improved evenwhen the first signal is multiplexed into the second signal.

All examples and conditional language recited herein are intended forpedagogical purposes of aiding the reader in understanding the inventionand the concepts contributed by the inventor to further the art, and arenot to be construed as limitations to such specifically recited examplesand conditions, nor does the organization of such examples in thespecification relate to a showing of the superiority and inferiority ofthe invention. Although the embodiments of the present invention havebeen described in detail, it should be understood that the variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A transmission device comprising a processor,wherein the processor executes a process comprising: allocating aninformation signal from among a plurality of information signals to asubcarrier in a plurality of subcarriers; generating a first signal bymodulating each of the information signals allocated to each of thesubcarriers; receiving a second signal; first acquiring firstinformation on the first signal and second information on the secondsignal, the first information including a transmission characteristic ofa frequency in a plurality of frequencies in the first signal and asignal level associated with the frequency, the second informationincluding a carrier frequency of the second signal; subtracting a firstsignal level of the first signal associated with the carrier frequencyof the second signal from a second signal level of the second signalassociated with the carrier frequency of the second signal; secondacquiring a signal level difference between the first signal level andthe second signal level; third acquiring a conversion signal level bysubtracting the signal level difference from a target signal level ofthe second signal, the target signal level corresponding to a signallevel tailored to a counterpart device; searching a target frequencycorresponding to the conversion signal level from the transmissioncharacteristic; shifting the carrier frequency of the second signal tothe target frequency searched by the searching, so that the subcarrierto which the information signal is allocated is different from thecarrier frequency; multiplexing the first signal and the second signalshifted by the shifting; and outputting the multiplexed signal.
 2. Thetransmission device according to claim 1, wherein the shifting includesshifting the carrier frequency of the second signal to the targetfrequency that is associated with the subcarriers on a high frequencyside.
 3. The transmission device according to claim 1, wherein theshifting includes shifting the carrier frequency of the second signal tothe target frequency that is associated with a free subcarrier in theplurality of subcarriers in the first signal.
 4. The transmission deviceaccording to claim 1, wherein the shifting includes shifting the carrierfrequency of the second signal to the target frequency of subcarrierother than the subcarriers in the first signal.
 5. The transmissiondevice according to claim 1, wherein the subtracting includes adjusting,when determining the signal level difference is less than apredetermined threshold, the second signal level of the second signal.6. The transmission device according to claim 1, wherein themultiplexing includes allocating the shifted carrier frequency of thesecond signal to the subcarrier of the target frequency to multiplex thefirst signal and the second signal shifted by the shifting.
 7. Atransmitter comprising a processer, wherein the processor executes aprocess comprising: allocating an information signal from among aplurality of information signals to a subcarrier in a plurality ofsubcarriers; generating a first signal by modulating each of theinformation signals allocated to each of the subcarriers; receiving asecond signal; first acquiring first information on the first signal andsecond information on the second signal, the first information includinga transmission characteristic of a frequency in a plurality offrequencies in the first signal and a signal level associated with thefrequency, the second information including a carrier frequency of thesecond signal; subtracting a first signal level of the first signalassociated with the carrier frequency of the second signal from a secondsignal level of the second signal associated with the carrier frequencyof the second signal; second acquiring a signal level difference betweenthe first signal level and the second signal level; third acquiring aconversion signal level by subtracting the signal level difference froma target signal level of the second signal, the target signal levelcorresponding to a signal level tailored to a counterpart device;searching a target frequency corresponding to the conversion signallevel from the transmission characteristic; shifting the carrierfrequency of the second signal to the target frequency searched by thesearching, so that the subcarrier to which the information signal isallocated is different from the carrier frequency; multiplexing thefirst signal and the second signal shifted by the shifting; andoutputting the multiplexed signal.
 8. A transmission method that causesa transmission device to perform a process comprising: allocating, by aprocessor of the transmission device, an information signal from among aplurality of information signals to a subcarrier in a plurality ofsubcarriers; generating, by the processor, a first signal by modulatingeach of the information signals allocated to each of the subcarriers;receiving a second signal; first acquiring, by the processor, firstinformation on the first signal and second information on the secondsignal, the first information including a transmission characteristic ofa frequency in a plurality of frequencies in the first signal and asignal level associated with the frequency, the second informationincluding a carrier frequency of the second signal; subtracting, by theprocessor, a first signal level of the first signal associated with thecarrier frequency of the second signal from a second signal level of thesecond signal associated with the carrier frequency of the secondsignal; second acquiring, by the processor, a signal level differencebetween the first signal level and the second signal level; thirdacquiring, by the processor, a conversion signal level by subtractingthe signal level difference from a target signal level of the secondsignal, the target signal level corresponding to a signal level tailoredto a counterpart device; searching, by the processor, a target frequencycorresponding to the conversion signal level from the transmissioncharacteristic; shifting, by the processor, the carrier frequency of thesecond signal to the target frequency searched by the searching, so thatthe subcarrier to which the information signal is allocated is differentfrom the carrier frequency; multiplexing, by the processor, the firstsignal and the shifted second signal; and outputting, by the processor,the multiplexed signal.